The objective of the present proposal is to investigate correlations between ionic fluxes mediated by the sodium pump on the one hand, and ionic, metabolic, and electrical parameters on the other hand. A thorough quantitative analysis of these correlations is prerequisite to a complete kinetic formulation of the operation of the sodium pump, and will help throw light on the molecular mechanisms underlying both the ion fluxes and the accompanying biochemical transformations, and on the thermodynamics of the intermediate steps. The sodium pump of animal cells is known to catalyze not only Na:K exchange, but also Na:Na exchange whenever intracellular (ADP) is raised; the former exchange is enhanced by external potassium, the latter inhibited. The kinetics of this complex behavior will be investigated, mainly on the squid giant axon, by means of the internal dialysis technique, which allows: 1) experimental control of the independent variables (i.e., electrical, metabolic, and ionic parameters) and 2) measurement of the dependent variables: Na and K in-and effluxes, ATP hydrolysis, 14C and 32P label exchanges, etc. Using proper stability constants for magnesium-nucleotide complexes, as well as equilibrium constants for various nucleotide equilibria (myokinase, phosphagen kinase), internal perfusion media will be prepared whose composition with respect to ATP, ADP, AMP, their respective Mg complexes, free Mg2 ion levels, and pH, will be exactly known. Conditions will be chosen such that either the Na:K exchange mode or the Na:Na exchange mode predominates, and a detailed kinetic analysis will be made of either mode with respect to the pertinent substrates and products (Na., K., Na., K ) and of the transitions and interrelationships between the modes of operation.